The present invention relates to a method for using the excess heat and mechanical energy available in foundry blast furnace gases and, more particularly, to a process for treating high temperature blast furnace exhaust gases that contain residual amounts of unburned hydrocarbon fuel to form a clean, supplemental combustion gas and working fluid for a gas turbine engine.
It is known that blast furnace operations that reduce iron ore to commercial grade iron and steel suffer from significant thermal inefficiencies. A typical blast furnace only uses about 66% of the total heat energy supplied to the metallurgical plant to produce crude iron, with significant quantities of air being required to burn the coke feed. It is also known that blast furnace exhaust streams often contain useful amounts of unburned hydrocarbons that must be released, treated or remain unused during the iron ore reduction process.
Past efforts to conserve heat energy generated during and after blast furnace operations have focused primarily on reducing the amount of fuel necessary to perform the initial foundry operation, such as reducing the amount of coke gas, natural gas or heating oil used to heat the iron ore. Other methods have attempted to recycle heat from the blast furnace exhaust gases to support the initial extraction of iron ore on the theory that the economics of any metallurgical process can be improved by increasing the input temperature of the fuel gas feed.
A limited number of techniques have also been proposed in recent years to improve the thermal efficiency of a foundry operation by making better use of the heat and potential work of the blast furnace exhaust in downstream operations, including gas streams driving rotating pieces of equipment that generate electricity. However, these known processes suffer from significant thermal inefficiencies, as well as operational problems due to the presence of solid particulates in the exhaust gas. In addition, for smaller size foundry furnaces, the electrical generating capacity of an associated gas turbine engine may not be economically justified.
The initial investment to purchase and install the required generating equipment can be prohibitively expensive, making the time to recover an investment in a downstream process unacceptably long. Many so-called “combined” plants also do not have sufficient space to accommodate the relatively complex and expensive equipment necessary to generate electricity from exhaust blast furnace gases primarily because of the need to accommodate large amounts of nitrogen introduced into the blast furnace in the form of ambient air at the outset of the foundry operation. A nominal exhaust gas stream from a conventional blast furnace contains about 45-50% by volume nitrogen.
Thus, to date, most conventional plants treating blast furnace exhaust gas cannot fully recapture the fuel value or potential work of the exhaust gas as it leaves the blast furnace. Typically, the gas emerges from the furnace at a pressure of between 1.5 and 2.0 bar gauge with a relatively high sensible heat content (gas temperatures typically range between 150° and 200° C.). The exhaust gases also contain nitrogen and a residual amount of unburned hydrocarbon fuel in gaseous form. Attempts have been made to recover the residual fuel components and/or to use the work force available from the exiting blast furnace gases. Invariably, however, such process have met with limited success because of the presence of nitrogen, carbon dioxide and carbon monoxide in the exhaust that tend to lower the fuel value for any later use, particularly in a gas turbine engine.
The potential therefore exists to increase the overall thermal efficiency of a combined fuel feed to a gas turbine engine, provided the amount of nitrogen fed to the turbine engine can be significantly reduced. In the past, molecular sieves have been used on a small scale to separate N2 from ambient air using commercially available “oxygen concentrators.” One prior method known as “pressure swing adsorption” (“PSA”) has been used successfully to separate a particular gas species from a mixture of gases under pressure based on the species' molecular characteristics and affinity for an adsorbent material. PSA operates at near-ambient temperatures and thus differs from cryogenic distillation techniques causing gas separation. Special adsorptive materials serve as a molecular sieve, preferentially adsorbing a target gas species at high pressure. The process then swings to low pressure to desorb the adsorbent material.
For various reasons, conventional oxygen concentrators employing PSA technology cannot be used effectively on a larger scale to treat blast furnace exhaust gases or to create a supplemental feed to a gas turbine engine. For example, conventional oxygen concentrators simply are not effective in removing other byproducts of blast furnace combustion, such as carbon monoxide, carbon dioxide or argon present in the blast furnace exhaust stream.
Exhaust gases from a foundry blast furnace pose another significant problem that makes the raw gas unacceptable for use in downstream gas turbine engines. The gas cannot be fed directly into the combustor or any stage of a power generating gas turbine engine due to the presence of solid particulates generated and entrained in the exhaust gas during the original blast furnace operation. Manufacturers of gas turbine engines normally require a maximum inlet particulate loading of about 5 mg/Nm3. Thus, before any blast furnace gas can qualify for use in a gas turbine engine, it must undergo cleaning in order to transform the gas from its initial “dirty” condition (typically 8.10 g/Nm3) to the standards required for a gas turbine inlet.